1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser which emits laser light in the direction perpendicular to a semiconductor substrate and a surface emitting semiconductor laser array using the surface emitting semiconductor laser.
2. Description of Related Art
A surface emitting semiconductor laser emits laser light in the direction perpendicular to a semiconductor substrate on which a resonator is formed in the perpendicular direction. The resonator causes laser oscillation to occur and emits laser light. The resonator is formed by depositing a reflection layer, an active layer, and a reflection layer, in that order, on the semiconductor substrate.
The surface emitting semiconductor laser has various excellent characteristics. Specifically, the surface emitting semiconductor laser can be easily arrayed (a plurality of lasers are arranged on one semiconductor substrate), has a small threshold, has a stable oscillation wavelength, exhibits an isotropic, small radiation angle in comparison with edge emitting lasers, and the like. Therefore, as a two-dimensionally integratable semiconductor laser, application of the surface emitting semiconductor laser to parallel optical communications, parallel optical arithmetic, laser beam printers, and the like has been expected.
In the case of forming optical devices using a semiconductor laser, devices such as a polarizer or a beam splitter are often used. The reflectivity of the polarizer and beam splitter are dependent on the polarization direction. In the case of using a semiconductor laser assembled into optical devices, if the polarization direction of laser light is insufficiently controlled, problems occur such as changes in the light intensity depending on the polarization direction. Therefore, it is important to control the polarization direction of laser light.
However, since the angle of radiation of laser light emitted from the resonator is isotropic because of the isotropic structure thereof, it is difficult to control the polarization direction of laser light. To deal with this problem, attempts to control the polarization direction of laser light in the surface emitting semiconductor laser has been made. For example, Japanese Patent Applications Laid-open No. 8-116130 and No. 6-224515 disclose technologies for controlling the polarization direction of laser light in the surface emitting semiconductor laser.
An object of the present invention is to provide a surface emitting semiconductor laser capable of controlling the polarization direction of laser light and a surface emitting semiconductor laser array.
According to the present invention, a surface emitting semiconductor laser in which a resonator is formed on a semiconductor substrate in the perpendicular direction, from which laser light is emitted in the direction perpendicular to the semiconductor substrate, comprises:
a columnar semiconductor deposit which comprises at least part of the resonator; and
an insulating layer formed in contact with the side of the semiconductor deposit;
wherein the insulating layer exhibits anisotropic stress caused by the planar configuration thereof, and the polarization direction of laser light is controlled by the anisotropic stress.
The anisotropic stress caused by the planar configuration means stresses which are anisotropic and induced by the planar configuration of the insulating layer. The anisotropic stress includes stresses in the directions intersecting at right angles in different amounts.
According to this surface emitting semiconductor laser, the anisotropic stress can be directly applied to an active layer included in the semiconductor deposit from the insulating layer. Specifically, since the insulating layer exhibits anisotropic stress caused by the planar configuration thereof, the anisotropic stress is applied to the active layer which is in contact with the insulating layer, thereby providing the gain of laser light with anisotropy. As a result, laser light in the specific polarization direction can be preferentially amplified, whereby the polarization direction of laser light can be controlled in a specific direction.
In the surface emitting semiconductor laser according the present invention, assuming that the axes intersecting at right angles through the center of the upper side of the semiconductor deposit are respectively the x-axis and y-axis, the anisotropic stress preferably includes stresses in the x-axial direction and the y-axial direction in different amounts.
According to the surface emitting semiconductor laser having the above configuration, since the anisotropic stress includes stresses in the x-axial direction and the y-axial direction in different amounts, the gain of laser light in the direction perpendicular to the direction in which the amount of stress is the greatest of the x-axial direction and y-axial direction becomes preferential, whereby laser light polarized in the direction perpendicular to the direction in which the amount of stress is greater can be obtained. Therefore, the polarization direction of laser light can be controlled. In addition, since the planar configuration of the insulating layer can be easily adjusted in the layer forming step, the polarization direction can be easily controlled. Moreover, in the case of fabricating a laser array, laser light with uniform characteristics can be obtained.
The following (1) to (4) can be given as embodiments of the insulating layer exhibiting anisotropic stress caused by the planar configuration thereof.
(1) Assuming that the axes intersecting at right angles through the center of the upper side of the semiconductor deposit are respectively the x-axis and y-axis, the planar configuration of the insulating layer may be designed so that the distance between the center and the first intersection point between the x-axis and the side of the insulating layer differs from the distance between the center and the second intersection point between the y-axis and the side of the insulating layer.
Since the planar configuration of the insulating layer is designed so that the distance between the center and the first intersection point between the x-axis and the side of the insulating layer differs from the distance between the center and the second intersection point between the y-axis and the side of the insulating layer, the stress applied to the active layer in the semiconductor deposit from the insulating layer exhibits anisotropy. As a result, the gain of laser light in the direction perpendicular to the direction in which the amount of stress is the greatest of the x-axial direction and y-axial direction becomes preferential, whereby laser light polarized in the direction perpendicular to the direction in which the amount of stress is greater can be obtained. Therefore, the polarization direction of laser light can be controlled.
According to a preferable embodiment of the surface emitting semiconductor laser, a surface emitting semiconductor laser in which a resonator is formed on a semiconductor substrate in the perpendicular direction, from which laser light is emitted in the direction perpendicular to the semiconductor substrate, comprises:
a columnar semiconductor deposit which comprises at least part of the resonator; and
an insulating layer formed in contact with the side of the semiconductor deposit;
wherein, assuming that the axes intersecting at right angles through the center of the upper side of the semiconductor deposit are respectively the x-axis and y-axis, the planar configuration of the insulating layer is designed so that the distance between the center and the first intersection point between the x-axis and the side of the insulating layer differs from the distance between the center and the second intersection point between the y-axis and the side of the insulating layer.
According to this surface emitting semiconductor laser, the above-described effects can be achieved.
(2) The planar configuration of the insulating layer may be symmetrical with regard to at least either the x-axis or y-axis.
In the surface emitting semiconductor laser, since the planar configuration of the insulating layer is symmetrical with regard to at least either the x-axis or y-axis, the stress applied to one side of the active layer in the semiconductor deposit and the stress applied to the other side symmetrical with regard to the symmetry axis among the x-axis and y-axis are equivalent and face 180 degrees in opposite directions. Therefore, anisotropy of the stress in the direction parallel to the axis intersecting the symmetry axis at right angles can be further increased. Moreover, polarization of laser light in the direction parallel to the axis intersecting the symmetry axis at right angles can be strictly controlled.
(3) The planar configuration of the insulating layer may be two-fold rotationally symmetrical around the semiconductor deposit.
Two-fold rotational symmetry means that the planar configuration of the insulating layer rotated around the semiconductor deposit at 180 degrees coincides with the original planar configuration of the insulating layer (one-fold is 90 degrees).
In the surface emitting semiconductor laser, since the planar configuration of the insulating layer is two-fold rotationally symmetrical around the semiconductor deposits the stress applied to one side of the active layer in the semiconductor deposit and the stress applied to the other side of the active layer are equivalent and face 180 degrees in opposite directions. Therefore, anisotropy of the stress in the direction parallel to the axis intersecting the above axis can be further increased. Because of this, the polarization direction of laser light can be controlled more strictly.
(4) The insulating layer may be embedded.
In the surface emitting semiconductor laser in which the insulating layer is embedded, since the anisotropic stress occuring in the insulating layer can be applied to the entire area of the semiconductor deposit including the active layer through the side of the semiconductor deposit, whereby polarization of laser light can be securely controlled.
In the surface emitting semiconductor laser, the insulating layer is preferably formed of a polyimide resin, acrylic resin, or epoxy resin.
In the case of forming an insulating layer using such a resin, the resin is cured by irradiating with energy rays such as heat or light. The resin shrinks significantly in volume during curing, thereby causing a large amount of tensile strain in the semiconductor deposit due to the volume shrinkage. Specifically, a large amount of tensile stress is applied to the semiconductor deposit including the active layer from the insulating layer. Because of this, a large amount of stress can be applied to the active layer, whereby the polarization direction of laser light can be controlled more stably.
In the surface emitting semiconductor laser according to the present invention, the planar configuration of the insulating layer may be rectangular, diamond-shaped, elliptical, or the like.
A surface emitting semiconductor laser array according to the present invention is formed by arraying a plurality of the above-described surface emitting semiconductor lasers according to the present invention.
Since this surface emitting semiconductor laser array is formed by arraying a plurality of surface emitting semiconductor lasers, a surface emitting laser array can be appropriately fabricated according to the application and purpose by forming a predetermined number of surface emitting lasers in a predetermined arrangement.